Impact of changing land use practices on nitrate export by the Mississippi River

Abstract

The increased use of nitrogen fertilizer in the Mississippi River Basin since the 1950s has been blamed for declining water quality, the degradation of aquatic ecosystems and the growth of a seasonal hypoxic zone in the Gulf of Mexico. In this study, we use the IBIS terrestrial ecosystem model and the HYDRA aquatic transport model to examine how agricultural practices and climate influenced terrestrial and aquatic nitrogen cycling across the Mississippi Basin and the nitrate export to the Gulf. The modeling system accurately depicts the observed trends and interannual variability in nitrate export by the Mississippi River (r2 > 0.83), and several of the major tributaries, between 1960 and 1994. The challenge of simulating nitrate export from the central western sub‐basins highlights the key role of processes like denitrification. The simulations demonstrate that three factors led to the doubling of nitrate export by the Mississippi River since 1960: (1) an increase in fertilizer application rates, particularly on maize; (2) an increase in runoff across the basin; and (3) the expansion of soybean cultivation. By the early 1990s, fertilized crops may have accounted for almost 90% of the nitrate leached to the river system, despite representing only 20% of the watershed area. The majority of the nitrate exported to the Gulf appears to originate from “hot spots,” including a stretch of the “Corn Belt” across Iowa, Illinois, and Indiana. The relative contribution of such heavily fertilized lands, particularly those in close proximity to higher order streams, can be even greater during wet years.